This invention is concerned with focal plane array technology for detecting incoming infrared electromagnetic radiation with a two dimensional array of detectors.
Many advanced optical sensors require spectral selectivity as part of the function of target detection and discrimination. In the infrared region, for example, various methods have been proposed to provide multicolor infrared detection, some making use of a complex detector architecture, others using spectral filtering methods. Multiple apertures, or the use of dichroic beamsplitters with separate focal planes, are among the optical solutions which have been devised. Alternatively, segmented filters are placed close to the focal plane. This last approach has the disadvantages of complicating the optical and mechanical design, increasing the cooling requirements, and introducing the possibility of spectral crosstalk. Current technology requires that such filters be deposited onto separate substrates because most optical thin films must be deposited onto heated substrates at temperatures well above the safe limits for HgCdTe infrared detectors, while filters deposited prior to detector formation must withstand subsequent processing procedures. Filters on separate substrates incur several system penalties. The filter is a separate component which must be cryogenically cooled, placing increased demands on system cooling capacity. In addition, multiple reflections can occur between the filter and the focal plane array, resulting in image degradation and spectral crosstalk. Another multicolor approach requires the use of a segmented focal plane with scanning capability to obtain accurate spatial as well as spectral information.
Spectral filter arrays are known in the art for visible wavelength applications. These filters consist of arrays of thin film interference filters, each color being determined by an independent filter design. The methods used to fabricate such a filter directly on a detector array cannot be utilized in the mid and long wavelength infrared because the required filter thickness for these portions of the spectrum is greater than the conventional photoresist thickness used in lithographic processes, making removal of the resist exceedingly difficult.
As a consequence of these restrictions, current infrared focal plane array technology is limited to a single wavelength band of operation for each detector array or subarray. Multicolor focal planes require separate staring arrays with individual spectral filters or are built up of individual linear array components (usually several elements in width), each associated with a different bandpass filter. Other multicolor approaches require scanning of the scene over multiple linear arrays with individual filter elements.
An improved optical thin film technology which would integrate spectral bandpass filters with an infrared focal plane array could have a major impact on future imaging and surveillance systems. New material and processing technology is needed to permit the deposition of spectral filters onto infrared arrays at ambient temperatures and to fabricate filter arrays directly on the focal plane arrays so that different regions of the same array can respond to different wavelengths. Furthermore, advanced designs and processing procedures are required to minimize the thickness of the deposited filter that must be spatially patterned to achieve the wavelength selection between adjacent pixels or focal plane segments.